Bumping electronic components using transfer substrates

ABSTRACT

A method for forming solder bumps on an electronic component. Providing a transfer substrate having a plurality of solder balls, disposing the transfer substrate on the surface of the electronic component, heating to reflow the solder balls onto the electronic component; and removing the sacrificial substrate. The transfer substrate may comprise a sacrificial film and a metal layer patterned with a mask which is used to form solder balls on the transfer substrate. Or, the transfer substrate may comprise a sheet of material having solder balls embedded at least partially in the sheet. A method of aligning a part being bumped with a transfer substrate, using a shuttle mechanism and an alignment film is disclosed.

CROSS-REFERENCE TO PENDING APPLICATIONS

This is continuation-in-part (CIP) of U.S. Ser. No. 10/989,107 filedNov. 15, 2004 (U.S. Pat. No. 7,288,471, Oct. 30, 2007).

This is also a continuation-in-part of commonly-owned, U.S. patentapplication Ser. No. 11/370,500 filed Mar. 7, 2006.

Ser. No. 10/989,107 is a non-provisional filing of Provisional PatentApplication No. 60/520,701 filed Nov. 17, 2003 by Mackay.

Ser. No. 10/989,107 is a continuation-in-part of commonly-owned, U.S.patent application Ser. No. 10/643,766 filed Aug. 18, 2003 (now U.S.Pat. No. 7,007,833), which is a continuation-in-part of:

-   -   Ser. No. 09/962,007 filed Sep. 24, 2001 (U.S. Pat. No.        6,609,652, Aug. 26, 2003; hereinafter referred to as the '652        patent) which discloses ball bumping substrates, particularly        wafers;    -   which is a continuation-in-part of:        -   U.S. Ser. No. 09/273,517 filed Mar. 22, 1999 (U.S. Pat. No.            6,293,456, Sep. 25, 2001),    -   which is a continuation-in-part of each of:        -   U.S. Ser. No. 08/863,800 filed 27 May 1997 (U.S. Pat. No.            5,988,487, Nov. 23, 1999);        -   U.S. Ser. No. 60/079,006 filed 23 Mar. 1998;        -   U.S. Ser. No. 60/079,221 filed 24 Mar. 1998; and        -   U.S. Ser. No. 60/092,055 filed 8 Jul. 1998,            all of which are incorporated in their entirety by reference            herein.

Ser. No. 10/989,107 is also a continuation-in-part of commonly-owned,U.S. patent application Ser. No. 10/630,310 filed Jul. 30, 2003,incorporated in its entirety by reference herein, which is acontinuation-in-part of the aforementioned U.S. Ser. No. 09/962,007filed Sep. 24, 2001 (U.S. Pat. No. 6,609,652, Aug. 26, 2003)

TECHNICAL FIELD OF THE INVENTION

The invention relates to methods of forming solder balls on (“bumping”)circuitized substrates which are electronic components such assemiconductor devices (integrated circuit chips) and interconnectionsubstrates, and to techniques for forming the solder balls on theelectronic components.

BACKGROUND OF THE INVENTION

In recent years, flip-chip bonding techniques have increasingly beenused to connect (bond) integrated circuit (IC) chips to interconnectionsubstrates and to package substrates. In flip-chip bonding an IC chipcomponent to an interconnection component such as ceramicinterconnection substrate, a plurality (e.g., an array) of solder balls(also called “solder bumps”) is formed on a face of a component,typically the IC chip component, and the bumped component is broughtinto a face-to-face relationship with the other component. The twocomponents are then heated (such as in a furnace) to reflow (heat, thenallow to cool) the solder bumps, thereby making electrical connectionsbetween respective terminals of the two components.

A “captured cell” technique is described in U.S. Pat. No. 6,609,652 (the'652 patent). For example, FIG. 1 of the '652 Patent, reproduced as FIG.1 herein, illustrates a technique 100 for forming solder balls on asurface of a substrate 102. The substrate 102 has number of pads 104 onits top (as viewed) surface. The pads 104 are typically arranged in anarray, having a pitch (center-to-center spacing from one another). Thesubstrate 102 is disposed atop a heater stage 106. A mask (stencil) 110is provided. The mask 110 is a thin planar sheet of relatively stiffmaterial, such as molybdenum, having a plurality of openings (cells)112, each corresponding to a pad 104 whereupon it is desired to form asolder ball on the substrate 102. The mask 110 is placed on the top (asviewed) surface of the substrate 102 with the cells 112 aligned over thepads 104. The cells 112 in the mask 110 are filled with solder material114. This is done in any suitable manner such as by smearing soldermaterial on the top (as viewed) surface of the mask 110 and squeegee-ingthe solder material 114 into the cells 112 of the mask 110. A pressureplate 120 is disposed onto the top (as viewed) surface of the mask 110.This holds the mask 110 down onto the substrate 102, and the substrate102 down onto the heater stage 106. This also closes off the cells112—hence, the terminology “captured cell”. The heater stage 106 isheated up, typically gradually, to a temperature sufficient to cause thesolder material in the cells 112 to melt (reflow). When the soldermaterial melts, the individual solder particles will merge (flow)together and, due to surface tension, will try to form (and, typically,will form) a sphere. When the solder material re-solidifies, it assumesa general spherical or hemispherical shape. The mask 110 is then removedfrom the substrate 102.

A mechanism for shuttling a mask from a printing station to a productbeing ball bumped is described in the '652 Patent. For example, FIG. 10of the '652 Patent, reproduced as FIG. 2 herein. illustrates anexemplary ball bumping machine 200 having a base 202, a chuck 204 on theleft side for holding a wafer 206 and a heater stage 208 on the rightside. A mask 210 is held in a frame 212. The chuck 204 is disposed inchuck base 214. The heater stage 208 is disposed in a heater stage base216. An elongate shuttle (carriage) mechanism 218 is pivotally attachedto the base 202 at a point “P” between the chuck 204 and the heaterstage 208. The frame 212 is held in a carrier 220 which is attached tothe opposite (free) end of the shuttle mechanism 218. A motor 221controls the position of the shuttle mechanism 218. The shuttlemechanism 218 can shuttle the mask 210 (i.e., the carrier 220) betweenthe heater stage 208 on the right side (as shown) and the chuck 204 onthe left side. The shuttle mechanism 216 pivots about the point “P”.Cameras (not shown) are used to make alignments, for example of the mask210 to the wafer 206. A set of holddown magnets 222, which preferablyare electromagnets, selectively hold the chuck base 214 to the machinebase 202. Similarly, a set of holddown magnets 222, which preferably areelectromagnets, selectively hold the heater stage base 216 to themachine base 202. The carrier 220 is ferrous, or has ferrous “lands”. Aset of lift magnets 226, which preferably are electromagnets,selectively hold the carrier 220 to the heater stage base 216.Similarly, a set of lift magnets 228, which preferably areelectromagnets, selectively hold the carrier 220 to the chuck base 214.In this manner, the mask can be brought down onto the heater stage, themagnets 226 turned on, the magnets 224 turned off, and the heater stagecan be lifted by the shuttle mechanism 216. In other words, when themask is shuttled, it can take the heater stage with it. Similarly, themask can be brought down onto the chuck, the magnets 228 turned on, themagnets 222 turned off, and the chuck can be lifted by the shuttlemechanism 216.

REFERENCES CITED

The following patents and published applications are incorporated byreference in their entirety herein:

U.S. Pat. No. 4,898,320 (Dunaway; 1990) discloses method ofmanufacturing a high-yield solder bumped semiconductor wafer. The methodcomprises providing a non-solderable transfer substrate having atransfer surface for receipt of solder material; depositing soldermaterial onto the transfer surface to form solder bumps in apredetermined pattern; aligning solderable conductive elements of asemiconductor wafer with the predetermined solder bumps on the transfersurface; and reflowing the patterned solder bumps into wetted contactwith the wafer conductive elements.

U.S. Pat. No. 6,153,505 (Bolde; 2000) discloses plastic solder arrayusing injection molded solder. A solder injection mold apparatus andmethod for providing solder balls to a printed circuit board substrateusing the solder injection mold apparatus in the plastic ball grid array(PBGA). The solder mold through holes are chamfered at entry and at exitends to assist in receipt of molten solder and the formation andtransfer of solder balls to lands on the substrate. A blind recess isprovided in the second major surface of the mold, i.e. the side facingthe substrate, in order to accommodate electronic components mountedthereon. Solder balls are delivered and metallurgically affixed to thelands in a process that requires only one reflow, leaving the throughholes clean of solder and the mold ready for reuse. The material ofwhich the substrate, mold and base plate are comprised is selected to benon-wettable by solder and mutually compatible with respect to CTE whenexposed to temperatures of molten solder.

U.S. Pat. No. 6,333,469 (Inoue; 2001) discloses wafer-scale packagestructure and circuit board attached thereto. A wafer-scale packagestructure in which a circuit board for rearranging electrode pads of awafer is laminated on the wafer integrally. The circuit board can bedivided into individual chip-size packages (CSPS) and which includes alayer of polyimide resin, and connection between the wafer and thecircuit board is performed by solder bump, while the circuit board isstuck on the wafer with an adhesive.

US 20050263571 (Belanger et al.; 2005) discloses injection moldedcontinuously solidified solder method and apparatus. A method andapparatus for forming solder bumps by molten solder deposition intocavity arrays in a substrate immediately followed by solidification ofmolten solder such that precise replication of cavity volumes isconsistently achieved in formed solder bump arrays. Various solderfilling problems, such as those caused by surface tension and oxidationeffects, are overcome by a combination of narrow molten Solder dispenseslots and solidification of dispensed molten solder.

US 20060035454 (Belanger et al.; 2006) discloses fluxless soldertransfer and reflow process. A process that permits the transfer andreflow of solder features produced by Injection Molded Solder (IMS) froma mold plate to a solder receiving substrate without the use of flux.Several embodiments produce solder transfer and reflow separately ortogether and use either formic acid vapor or partial concentration ofhydrogen, both in nitrogen, as the oxide reducing atmosphere. A finalembodiment produces fluxless transfer and reflow in only nitrogenthrough the use of ultrasonic vibration between the solder filled moldplate and solder receiving substrate.

BRIEF DISCLOSURE (SUMMARY) OF THE INVENTION

It is an object of the invention to provide an improved process forforming solder balls on electronic components.

The present invention advantageously utilizes various of the techniquesset forth in the '652 Patent, as well as in other commonly-owned patentsand pending applications, and presents a new method for ball bumping asubstrate. All of the matter that is discussed in previouscommonly-owned patents and pending applications is incorporated byreference herein.

According to the invention generally, a bump transfer substrate isprepared (bumped) with the proper alloy, bump size, and layout allowingthe end user to transfer the solder (bumps) onto his products withouthigh capital investments or large NRE (non-recurring) costs for smallvolumes. The technique is compatible with BGA (ball grid array), PCB(printed circuit board) pads, CSP (chip scale packaging), wafer levelCSP, flip-chip, and the like.

The bump transfer substrate is essentially a sacrificial substrate. Itmay be bumped using the captured cell technique described in the '652Patent (refer to FIGS. 1 and 2, above). Many advantages can accrue tofirst bumping a sacrificial substrate, then transferring the solderballs formed from the sacrificial substrate to a product.

According to the invention, a method for forming solder bumps on anelectronic component comprises the steps of: providing a transfersubstrate comprising a sacrificial substrate and plurality of solderballs; disposing the transfer substrate on the surface of the electroniccomponent; heating to reflow the solder balls onto the electroniccomponent; and removing the sacrificial substrate.

The transfer substrate may comprise a polyimide film, preferablysubstantially see-through, such as Kapton (™) covered with a layer ofmetal, such as copper. The layer of metal is patterned using a mask. Thesolder balls are formed on the transfer substrate, and this can be usingthe same mask.

The transfer substrate is urged against the electronic component, usinga pressure plate or air pressure. A sheet of resilient material may bedisposed between the pressure plate and the transfer substrate.

The sacrificial substrate may be removed either before or after cooling.

Alternatively, the sacrificial substrate comprises a sheet of siliconerubber; and the solder balls are at least partially, such as at least50% embedded in the sheet of silicone rubber.

The silicone rubber with embedded solder balls embodiment of transfersubstrate may be formed by: providing a blind hole mask having a planarsubstrate and a plurality of cells extending into a surface of thesubstrate; filling the cells with solder paste; reflowing the solderpaste to form solder balls; encapsulating the mask with a curableresilient material (silicone rubber); allowing the curable material tocure, forming a sheet; and removing the curable resilient material fromthe mask, with the solder balls least partially embedded therein.

According to the invention, a method of aligning a transfer substrate toa product being bumped comprises the steps of: providing a machinehaving first a chuck for holding the product being bumped, a secondchuck for holding a transfer substrate, and frame for holding analignment film; shuttling the alignment film to the product beingbumped, and aligning the product being bumped; and shuttling thealignment film to the transfer substrate, and aligning the transfersubstrate.

Then, for bumping the product, removing the alignment film, shuttlingthe transfer substrate to the product being bumped, urging the transfersubstrate against the product being bumped, supplying heat to reflowsolder balls from the transfer substrate to the product being bumped,and releasing the transfer substrate.

Other objects, features and advantages of the invention will becomeapparent in light of the following description thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made in detail to preferred embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. The drawings are intended to be illustrative, not limiting.Although the invention will be described in the context of thesepreferred embodiments, it should be understood that it is not intendedto limit the spirit and scope of the invention to these particularembodiments.

The structure, operation, and advantages of the present preferredembodiment of the invention will become further apparent uponconsideration of the following description taken in conjunction with theaccompanying drawings (FIGS.).

FIG. 1 is an exploded cross-sectional view of a method and apparatus forforming solder balls on substrates, as set forth in FIG. 1 of the '652Patent.

FIG. 2 is a schematic side view of a ball bumping machine, as set forthin FIG. 10 of the '652 Patent.

FIG. 3 is a cross-sectional view of a fixtureless transfer mechanism,according to the invention.

FIG. 4 is a cross-sectional view of a precision tool transfer mechanism,according to the invention.

FIGS. 5A, 5B and 5C are cross-sectional views of an embodiment of theinvention.

FIG. 6A is a side cross-sectional view illustrating a “composite” mask,according to the invention.

FIG. 6B is a side cross-sectional view illustrating a “bridge the gap”feature of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

U.S. Pat. No. 5,988,487 discloses methods for forming solder bumps onintegrated circuit chips (and other similar circuitized units). Ascreening stencil is laid over the surface of the substrate and solderpaste material is deposited into the stencil's apertures with ascreening blade. The stencil is placed in such a manner that each of itsapertures is positioned over a substrate pad, upon which a solder bumpis to be formed. Next, a flat pressure plate is laid over the exposedtop surface of the stencil, which creates a fully enclosed or“captured”, cell of solder paste within each stencil aperture. Then,with the stencil and plate remaining in place on top of the substrate,the substrate is heated to a temperature sufficient to reflow the solderpaste material. After reflow, the substrate is cooled, and the pressureplate and stencil are thereafter removed, leaving solder bumps on thesubstrate. The use of the pressure plate ensures the proper formation ofthe solder bumps at high densities of solder bumps (i.e., high densitiescorresponding to small solder bump sizes and small pitch distancesbetween solder bumps).

In U.S. Pat. No. 5,988,487, a method of forming solder bump structureson corresponding pads of a substrate comprises the steps of:

-   -   (a) laying a stencil over the surface of the substrate, the        stencil having apertures, said stencil being laid in such a        manner that an aperture is positioned over a substrate pad;    -   (b) screen depositing a solder paste material into the stencil        apertures;    -   (c) thereafter laying a flat plate over the exposed top surface        of the stencil; and    -   (d) thereafter heating the substrate with the stencil and plate        on top thereof to a temperature sufficient to reflow the solder        paste material.

In U.S. Pat. No. 5,988,487, a method of forming solder bump structureson corresponding pads of a substrate further comprises the steps of:

-   -   (e) thereafter lowering the temperature of said substrate to a        temperature point below 95% of the melting point of the reflowed        solder, as measured in degrees Kelvin, and above 80% of said        melting point; and    -   (f) thereafter lifting the plate and stencil from said substrate        at this temperature point.

In U.S. Pat. No. 5,988,487, a method of forming solder bump structureson corresponding pads of a substrate further comprises the steps of:

-   -   (a′) the stencil having apertures which correspond one-to-one        with corresponding pads of the substrate.

U.S. Pat. No. 6,609,652, discloses a mask (stencil) having cells(openings) is disposed on a surface of a heater stage, and is thenfilled (printed) with solder paste. Then a substrate is assembled to theopposite side of the mask. Then the solder paste is reflowed. This maybe done partially inverted. Then the mask is separated from thesubstrate, either before or after cooling. Solder balls are thus formedon the substrate, which may be a semiconductor wafer. A method forprinting the mask with solder paste is described.

In U.S. Pat. No. 6,609,652, a ball bumping machine comprises a machinebase, a chuck for holding a substrate, a heater stage, and a frame forholding a mask, and the ball bumping machine is characterized by:

-   -   the chuck is disposed on one side of the machine base;    -   the heater stage is on an opposite side of the machine base; and    -   means for shuttling the frame between the heater stage and the        chuck.

U.S. patent application Ser. No. 10/643,766 (U.S. Pat. No. 7,007,833)discloses a mask (stencil) having cells (openings) is disposed on asurface of a heater stage, and is then filled (printed) with solderpaste. Then a substrate is assembled to the opposite side of the mask.Then the solder paste is reflowed. This may be done partially inverted.Then the mask is separated from the substrate, either before or aftercooling. Solder balls are thus formed on the substrate, which may be asemiconductor wafer. A biased chuck urges the substrate into intimatecontact with the mask. A method for printing the mask with solder pasteis described. Methods of forming high aspect ratio solder bumps(including balls and reflowable interconnect structures) are described.

In U.S. patent application Ser. No. 10/643,766 (U.S. Pat. No.7,007,833), a method for forming solder bumps on a substrate having aplurality of pads on a surface thereof, comprises:

-   -   providing a mask having a first surface and a second surface,        and a plurality of cells extending from the first surface at        least partially through the mask to the second surface thereof;        filling the cells with solder paste; disposing the substrate in        a biased chuck assembly;    -   disposing the mask on the surface of the substrate with the        first surface of the mask adjacent the surface of the substrate;        urging the substrate into positive contact with the mask so as        to maintain substantially intimate contact between the first        surface of the mask and the surface of the substrate;    -   reflowing the solder paste; and    -   separating the substrate from the mask.

In U.S. patent application Ser. No. 10/643,766 (U.S. Pat. No.7,007,833), a chuck assembly for holding a semiconductor wafer inintimate contact with a mask, comprising a rigid, generally planar chuckbase, is characterized by:

-   -   a central recess extending into the stage from a top surface        thereof, said recess sized and shaped to receive a generally        planar, flexible diaphragm, said diaphragm extending across the        recess.

The above inventions (U.S. Pat. No. 5,988,487; U.S. Pat. No. 6,609,652;U.S. Pat. No. 7,007,833) relate to methods of forming solder balls onsubstrates which are electronic components such as semiconductor devices(integrated circuit chips) and interconnection substrates, and toapparatuses for forming the solder balls on the electronic components.Very generally, and this was done “directly” by filling cells of a maskwith solder paste, placing the filled mask on the electronic componentbeing bumped, reflowing the solder paste, and removing the mask. (Manymore specifics such as closed cell are involved.)

The present invention advantageously utilizes many of the techniquesdisclosed in Dockets M-1, M-3, M-6, but rather than bumping anelectronic component directly, a “sacrificial substrate” is bumped, andthe bumped sacrificial substrate, or “transfer substrate” is used tobump the electronic component.

As used herein, the verb “bump” means to form solder balls on, and thenoun “bump” means a solder ball.

Basic Transfer Substrate Manufacturing, Generally

According to the invention, generally, rather than bumping an electroniccomponent, first a sacrificial substrate (bump transfer substrate) isbumped, then the bumps (solder balls) are transferred to the electroniccomponent—namely, to corresponding pads on the electronic component.

The bump transfer substrate may be a film sheet, wafer, or substratewhich is coated with a base thin film (layer) of metal. The film can bea continuous (not patterned), blanket deposition. Utilizing the a waferbumping process, such as disclosed in the '652 Patent, a very thin layerof copper, or equivalent can be soldered to. (As described below, thelayer gets patterned.) This is due to the unique methods of interferenceliquid sphere attachment described in the '652 Patent. This allows thebump to consume this thin film and release from the substrate when theuser heats to the suggested re-flow profile.

A wafer bump mask is produced from etching, laser cutting, or additivemanufacturing. It is noted that this mask when used with the methoddescribed in the '652 Patent, hole size variations and positions do notrequire exact precision. The final product however produces excellentbump height uniformity, and a high tolerance for mismatches in materialexpansions. The typical mask costs are equivalent to a SMT (surfacemount technology) mask.

The bump mask can be similar to the masks that were used (in U.S. Pat.No. 5,988,487; U.S. Pat. No. 6,609,652; U.S. Pat. No. 7,007,833) forbumping the electronic component. Here, the mask is used to form bumpson the transfer substrate, but first can be used to pattern the metalfilm (layer) on the transfer substrate, thereby advantageously serving a“dual” purpose.

This bump mask is then used to photo process the thin film (metal layer)into pad defined patterns. (The pads are processed using the bump maskas a shadow mask for photoresist coat, followed by etching.) It may alsobe used with liquid photo-imageable solder masks, as a mask defined padpattern. This typically depends on the bump size and pitch requirements.This greatly reduces processing costs requiring only low cost laser filmmasks produced form auto cad, or Gerber files.

This produces a transfer substrate with the bump(s) attached, fluxcoated, ready to be transferred/released at the point of use. Thismethod produces transfer substrates for about $500 tooling cost andsubstrate cost of $5-7 and turn around times of 2-3 days are possible.Allowing a substantial markup while providing customers exceptional costsavings and time to market solutions.

The process of the present invention has few limitations. Any ball sizeor alloy can be produced, this bridges the gap presently experienced bythe industry as ball drop techniques are limited to about 10 mils andflip chip manufactures mostly have a hard time producing ball sizesgreater than 5 mils.

EMBODIMENT 1 Fixture-Less Transfer

Reference FIG. 3. Fixture-less transfers utilize a transfer substratewhich is a film, which is metallized. A typical film substrate materialis polyimide material (such as Kapton-™), having a thickness of 0.003in, and is preferably transparent or translucent (the sacrificialsubstrate is substantially see-through). The film itself is a“sacrificial film substrate”. A metal film such as copper (material) isdeposited on one surface of the film substrate. The metal thickness isin the range of 500-3000 Å, such as approximately 1000 Å.

First, the transfer substrate (metallized film substrate) is patterned(such as by using the bump mask, as described above) and bumped (again,using the bump mask). Typically, this would be done bump-up (bumping thetop, metallized surface of the transfer substrate). The transfersubstrate (bumped film substrate) can be set aside until needed, orshipped to a customer. This is one of the advantages of the invention.The bumping service provider does not necessarily need to handle (andrisk damaging) the product (electronic component). The transfersubstrate can be fluxed, now or later.

The thickness of the metal film on the film substrate should be thickenough that it is not consumed in the step of bumping the filmsubstrate. Later, when reflowing the bumps from the transfer substrateto the electronic component, as described hereinbelow, the metal filmwill be consumed, thereby also releasing the balls from the filmsubstrate, so it cannot be too thick. Exemplary ranges for thedimensions are set forth above.

Then, the bump prepared film (transfer substrate) and flux is inverted(bump-down) and aligned visually through the film with an underlingproduct being bumped (electronic component). Typically, the flux istacky enough to maintain alignment between the transfer substrate andthe electronic component. This can be done on a hot plate (not yetheated, of course). If visual alignment is not possible, otherconventional alignment means can be employed.

Next, a cover plate stack is placed on the top of the aligned film. Thecover plate stack comprises;

-   -   a sheet of resilient material, such as conformal silicone rubber        (silicone rubber is a synthetic elastomer made from a        cross-linked polymer which is reinforced with silica); and    -   a pressure plate—such as a quartz disk.

The conformal silicone rubber suitably has a thickness of from one-eightto one-quarter of an inch (0.125-0.250 inches). This sheet of siliconerubber serves various purposes:

-   -   it acts as a thermal barrier so that the pressure plate does not        “sink” heat which is being provided by the hot plate (FIG. 3) to        reflow the solder balls on the transfer substrate; and    -   is equalizes pressure and reduces stress concentrations such as        may result from an uneven topology across the top surface of the        product (electronic component) being bumped (FIG. 3)

It is within the scope of the invention that a manifold delivering airpressure to the back side (top, as viewed in FIG. 3) of the siliconerubber is used in lieu of a pressure plate to hold the transfersubstrate (bump prepared film) securely to the product being bumped(electronic component). The use of air pressure rather than a pressureplate is also pertinent to Embodiment 3, discussed hereinbelow. Thepressure plate (or air pressure mechanism) urges the transfer substrateagainst the electronic component.

Next, the cover plate stack (pressure plate and silicone rubber) isplaced atop the product/film on a cool (not yet heated) hot plate. Thenthe hot plate is heated to reflow the solder bumps on the transfersubstrate). As mentioned hereinabove, this is when the copper film (padsunder the balls) on the transfer substrate gets consumed, releasing theballs.

In this example, heating takes place through the electronic component,and should therefore be controlled so as to not damage the component.Some of the techniques described (in U.S. Pat. No. 5,988,487; U.S. Pat.No. 6,609,652; U.S. Pat. No. 7,007,833) could be employed to minimizeheating the electronic component.

Then the cover plate stack (pressure plate, silicone rubber; or pressuremanifold and silicone rubber) is removed. This can be done before orafter cooling.

Then the sacrificial film substrate can be peeled off, either hot orcold (before or after cooling).

If the sacrificial film substrate is removed before cooling, nicespherical will result bumps on the electronic component (product beingbumped).

If the sacrificial film substrate is removed after cooling, there is noneed to handle hot materials. However, bump distortions (for example,flat top bumps) may occur. But, as may be desirable to some clients, asecond re-flow would reform (reflow) the bumps as spheres, if required.

With respect to bumping the electronic component, the process of FIG. 3could be done inverted (turn the drawing sheet upside down to visualizethis), as discussed for example in the '652 Patent.

EMBODIMENT 2 Precision Tool Transfer

Reference FIG. 4. A precision tool modeled after (based on,substantially similar to, functionally comparable to) the FIG. 10 of the'652 Patent (FIG. 2, herein) would perform all of the needed functionsto transfer bumps from transfer (sacrificial) substrates onto products(electronic components). This would include alignment functions, contactand re-flow, and separation.

The unit has two chucks one for holding the product to be soldered(bumped) and one for holding the transfer substrate, both have flexheater inserts. A frame (in the shuttle mechanism) holds the film foralignment. This is copy of the film used to manufacture the mask andcosts about $10-$20.

As in the '652 Patent, means are provided (shuttle mechanism) forshuttling the frame between two positions—namely, between the productbeing bumped and the transfer substrate.

The alignment film is rotated (shuttled) to the product being bumped andalignment using knobs is accomplished. The film is then rotated(shuttled) to the transfer substrate which is then aligned.

The alignment film is then removed and the transfer substrate (in any ofthe embodiments set forth herein) is rotated (shuttled) directly overthe product. Pressure is then supplied to mate the bumps on the transfersubstrate to the pads on the electronic component. (The transfersubstrate is urged against the product being bumped) Heat is supplied,solder balls are reflowed onto the pads of the product being bumped, andthe transfer substrate is released.

The system can then separate and cool or cool in place as desired.

Using the alignment film provides a cheap way using no cameras toachieve appropriate alignments. A low cost laser film is produced. Theoperator loads this into the frame that swings back and forth betweenthe transfer substrate and the wafer (product being bumped), a visualalignment is made at wafer, then swing back to the transfer substrateand align, then remove the alignment film and swing (shuttle) thetransfer substrate to the wafer and heat (reflow), then swing thetransfer substrate away (before or after cooling, as describedhereinabove). Also, by using the alignment film, the transfer substratedoes not need to be transparent (e.g., kapton), as in the previousembodiment.

EMBODIMENT 3 Silicone Rubber Transfer Substrate

In this embodiment, a “blind hole” mask is used. A blind hole mask isdiscussed, for example, in the '652 Patent. In the '652 Patent (seeFIGS. 5A and 5B), cells (512) in the blind hole mask (520) were filledwith solder paste (514), then reflowed with the electronic component(302) in place. The mask material is suitably molybdenum (moly).

In this embodiment of the present invention, generally, the solder isreflowed in the blind hole mask, forming solder balls, the solder ballsare captured to a substrate, then later transferred to the electroniccomponent (product being bumped).

With reference to FIG. 5A, first a blind hole mask 500 is created whichis a planar substrate 502 having two opposite sides (top and bottom, asviewed), and a plurality of holes (cells) 504 extending into thesubstrate from on side (surface) thereof towards the other side thereof.Next, the holes are filled with solder paste, and reflowed to formsolder balls 506. The substrate 502 is suitably molybdenum (moly). Thesolder will not wet to the moly. Suitable exemplary dimensions are:

-   -   h, thickness of the mask, approximately 8 mils (0.008 inches)    -   d, depth of the cells, approximately 6 mils    -   w, cross-dimension of a cell, approximately 16 mils    -   s, spacing (pitch) of the cells, approximately 20 mils    -   D diameter of a solder ball, approximately 11.8 mils (0.3 mm)

A word about the figure (FIG. 5A). In reality, the cells 504 are not sosquare as illustrated. Actually, they are rounded, as a result ofetching. Including dished out somewhat at the bottom. This is normal.

In any case, it will be noted that the resulting solder balls 506 stickup, significantly, out of the cells 504 in the mask 500. It should alsobe remembered that when solder paste forms solder balls, about half theoriginal volume is lost due to volatiles etc.

FIG. 5B shows a next step. The blind hole mask 500 was used to createindividual, organized spheres (solder) 506 of a well-defined size at awell-defined pitch. It should then be allowed to cool down, but canstill be warm. As shown here, the front surface of the mask 500 isencapsulated a curable material 510, such as silicone rubber, which isallowed to cure. The silicone rubber forms a sheet. The top surface isshown a little wavy just to indicate that when applied, it is a liquid,and when cured it will be generally planar. Any curable material whichcan later be applied in substantially liquid form, cure to a resilientsheet, and can be peeled away will work here. RTV (room temperaturevulcanize) silicone rubber materials are a good choice.

A word about the figure (FIG. 5B). In reality, the silicone rubbermaterial 510 would enter the cells 504 in the gap between the spheres506 and the walls of the cell 504. For example, rather than encompassingonly about half of the sphere, the silicone rubber would encompassapproximately 75% of the sphere—namely, more than half (50%) but not100%. 60%-80% is adequate. (The figure is highly stylized. In reality,the bottoms of the cells are dished, and the solder balls will always bepoking at least slightly out of the rubber sheet.)

Then, as shown in FIG. 5C the sheet of cured silicone rubber 510 hasbeen peeled away (removed) from the mask (not shown in this figure), andit has solder balls 506 sticking out of its surface, the solder ballsbeing of a well-defined size at a well-defined pitch. (The solder ballsstick (adhere lightly) to the sheet of silicone rubber, also thesilicone rubber captures the balls by encompassing more than half of thesphere, yet is removable.) This is a saleable product, like the otherball bumped transfer substrates discussed herein, ready to ship to anelectronic component manufacturer who wants to bump their productwithout significant equipment costs and problems.

A word about the figure (FIG. 5C). This presents a more realistic viewwhere the balls 506 are about 75% buried (only 25% poking out) of thesilicone rubber sheet 510.

The sheet 510 with balls 506 is then used as a transfer substrate, asdescribed above. First aligned, using any suitable alignment technique,as described above. Then held securely to the product being bumped usingeither a pressure plate or an air pressure manifold, as described above.Then remove the rubber. (Silicone rubber will peel away easily, becauseit is resilient, and where it wraps around the equators of the spheres,it will yield.) Particularly with this method (silicone rubber sheetwith solder balls embedded), the use of air pressure (rather than asolid pressure plate) offers constant force even on non-flat substrates(being bumped).

This method (solder balls embedded in a sheet of silicone rubber) doesnot require a thin films on the transfer substrate, as in thepreviously-described embodiments.

In all embodiments, the solder wets to pads on the product being bumped,and said pads should of course have UBM (under bump metal) which is notconsumed when reflowing the solder balls from the transfer substrate tothe product being bumped.

The invention is useful for virtually any size and arrangement of balls.Custom arrangements can be formed. Or, standard pitch grids can bemass-produced, for example:

-   -   0.3 mm balls on 0.5 mm pitch (such as for chip scale)    -   5 mm balls on 8 mm pitch (such as for BGA package)        Regarding the Transfer Substrate

As discussed hereinabove, the bump transfer substrate is essentially asacrificial substrate, and may be bumped using the captured celltechnique described in the '652 Patent.

The transfer substrate may comprise a polyimide film, preferablysubstantially see-through, such as Kapton (™).

The transfer substrate may be urged against the electronic component,using a pressure plate or air pressure. A sheet of resilient materialmay be disposed between the pressure plate and the transfer substrate.

The sacrificial substrate may be removed either before or after cooling.

Alternatively, the sacrificial substrate comprises a sheet of siliconerubber; and the solder balls are at least partially, such as at least50% embedded in the sheet of silicone rubber.

The silicone rubber with embedded solder balls embodiment of transfersubstrate may be formed by: providing a blind hole mask having a planarsubstrate and a plurality of cells extending into a surface of thesubstrate; filling the cells with solder paste; reflowing the solderpaste to form solder balls; encapsulating the mask with a curableresilient material (silicone rubber); allowing the curable material tocure, forming a sheet; and removing the curable resilient material fromthe mask, with the solder balls least partially embedded therein.

A blind hole mask is discussed, for example, in the '652 Patent. In the'652 Patent (see FIGS. 5A and 5B), cells (512) in the blind hole mask(520) were filled with solder paste (514), then reflowed with theelectronic component (302) in place. The mask material is suitablymolybdenum (moly).

In the present invention, generally, the solder may be reflowed in theblind hole mask, forming solder balls, the solder balls are captured toa substrate, then later transferred to the electronic component (productbeing bumped).

FIGS. 6A and 6B correspond to FIGS. 5A and 5B of the aforementioned U.S.Pat. Nos. 6,293,456 and 6,609,652.

Composite Mask and Pressure Plate

The benefit of using a pressure plate to capture the solder material inthe cells of the mask has been discussed hereinabove. It is generallypreferred that the pressure plate be intimately held against the mask sothat there are no gaps for leakage, particularly when reflowing invertedor partially inverted.

According to an aspect of the invention, a composite mask performing thefunctions of a mask and a pressure (contact) plate are formed as anintegral unit, thereby assuring no leakage between the two.

FIG. 6A illustrates an embodiment of a composite mask 600. The compositemask 600 is a rigid planar structure having two portions—a mask portion610 comparable (e.g.) to the mask 110 described hereinabove, and apressure plate portion 620 comparable to the pressure plate 120described hereinabove. A plurality of cells 612 (compare 112) extendfrom a one surface of the composite mask 600, through the mask portion610, to the pressure plate portion 620. These “blind hole” type openings612 are filled with solder material 614 (compare 114) in the mannerdescribed hereinabove.

The composite mask 600 is suitably formed of a sheet of metal, such asmolybdenum, which is etched to have cells 612 extending into a surfacethereof (but not all the way through the sheet). Alternatively, thecomposite mask 600 can be formed from a sheet of metal comprising thepressure plate portion 620, a surface of which is masked, patterned, andplated up to form the mask portion 610 (with cells 612).

Alternatively, a composite-type mask can be formed from a discrete maskwelded or otherwise intimately joined (including adhered) to a discretepressure plate.

Bridging a Gap

An interesting feature/capability of the present invention isillustrated in FIGS. 6A and 6B, but is not limited to the use of acomposite mask. The composite mask 600 is illustrated disposed beneath asubstrate which is in an inverted position, for example the substrate302 from FIG. 3A in the '652 Patent (see also FIG. 3E). Note that nopart of the substrate 302 actually is in contact with the composite mask600—rather, that there is a small gap 624 between the opposing faces ofthe substrate and the mask.

As best viewed in FIG. 6B, when the solder material 614 reflows andforms a ball, the ball has a diameter (height) which is greater than thethickness of the mask (in this illustrative case, greater than thethickness of the mask portion 610 of composite mask 600), so it sticksout of the mask, “bridges” the gap 624, and wets itself to the pad 304on the substrate 302. The solder ball does this while it is in a liquidstate, at which point the mask can easily be separated from thesubstrate, thereafter allowing the solder ball to cool off (solidify).

Although, for example, the cells in the blind hole mask 600 are shown asbeing rectangular in cross-section, they could have the shapes, such ashemispherical.

As discussed hereinabove (with reference to FIG. 5A), a blind hole mask500 may a planar substrate 502 having two opposite sides (top andbottom, as viewed), and a plurality of holes (cells) 504 extending intothe substrate from on side (surface) thereof towards the other sidethereof. Next, the holes may be filled with solder paste, and reflowedto form solder balls 506. The substrate should be non-wettable. Thecells 504 may not be so square as illustrated. They may be rounded. Theresulting solder balls 506 stick up, significantly, out of the cells 504in the mask 500.

According to a feature of the invention, the transfer substrate is asubstantially planar sheet having a substantially planar front surface,a thickness, and a back surface which may be substantially parallel tothe front surface. The transfer substrate may comprise a sheet of anyrigid or semi-rigid, preferably “non-wettable” material, such as metal,molybdenum, silicone rubber, polyimide film, ceramic, quartz, glass.

Generally, the transfer substrate may be considered to be a“sacrificial” substrate in that it's primary purpose is to provide amedium upon which solder masses (such as solder balls) may be formed, atpre-defined locations, later (subsequently) to be transferred to acircuitized substrate such as an electronic component. The transfersubstrate itself need not be circuitized, and may or may not be reusable(after having been used for bumping a circuitized substrate, usableagain (and again) for bumping additional circuitized substrates). In thecase, for example, of a transfer substrate formed of a glass sheet, thetransfer substrate may readily be reusable.

According to a feature of the invention, the transfer substrate may haveholes or depressions extending only partially through the substrate intothe front surface thereof, and the holes or depressions may have anysuitable geometry, including dished out, hemispherical, and the like.

According to a feature of the invention, solder balls formed in thetransfer substrate may be partially embedded (disposed in the holes orrecesses) in the surface of the substrate, and extend at least partiallyout of the surface of the substrate.

According to a feature of the invention, after applying the transfersubstrate to the circuitized substrate, and heating to form solder ballson the circuitized substrate, the transfer substrate may be removedwhile the solder balls are still in liquid form (melted), or after thesolder balls have solidified on the circuitized substrate, or when thesolder balls are partially solidified.

According to a feature of the invention, inasmuch as the transfersubstrate may be considered to be a “sacrificial” substrate, thetransfer substrate may be left in place until the solder balls havesolidified upon the circuitized substrate and then, rather than simplylifting off the transfer substrate, the transfer substrate may first bepartially or fully dissolved (such as with a suitable solvent). This canhelp to release the solder balls from the transfer substrate, and can beused in cases where the solder balls tend to stick (such as by wetting,cohesion surface tension or other forces) to the transfer substrate.

According to a feature of the invention, in any instance where “solderpaste” is mentioned hereinabove, it should be understood that solderwithout flux can be used. As mentioned in the aforementioned U.S. Pat.Nos. 6,293,456 and 6,609,652, for example, although a solder materialcomprising solder particles and flux is described, the solder materialmay be dry, such as fluorine-treated, or using a forming or reducinggas. (column 24, line 64)

According to a feature of the invention, a blind hole mask such as hasbeen described, with solder balls extending from recesses in a surfacethereof, may be used as the transfer substrate itself (rather than tobump a separate transfer substrate), for bumping an electronic component(or circuitized substrate). And, when using the blind hole mask (withsolder balls formed therein) as a transfer substrate, the electroniccomponent may be bumped without using conventional flux. For example,various other fluxless solder transfer and reflow processes may be usedthat permit the transfer and reflow of solder bumps from the transfersubstrate to a circuitized substrate, without the use of flux. Forexample, producing solder transfer and reflow separately or together,and using either formic acid vapor or partial concentration of hydrogen,both in nitrogen, as the oxide reducing atmosphere. Or, producingfluxless transfer and reflow in only nitrogen through the use ofultrasonic vibration between the solder filled mold plate and solderreceiving substrate. Any chemical, mechanical, including ultrasonic,inert, nitrogen or hydrogen gas and any other “fluxless” method may beused.

Although the invention has been illustrated and described in detail inthe drawings and foregoing description, the same is to be considered asillustrative and not restrictive in character—it being understood thatonly preferred embodiments have been shown and described, and that allchanges and modifications that come within the spirit of the inventionare desired to be protected. Undoubtedly, many other “variations” on the“themes” set forth hereinabove will occur to one having ordinary skillin the art to which the present invention most nearly pertains, and suchvariations are intended to be within the scope of the invention, asdisclosed herein.

1. Method of aligning a transfer substrate to a product being bumpedcomprising the steps of: providing a machine having first a chuck forholding the product being bumped, a second chuck for holding a transfersubstrate, and frame for holding an alignment film; shuttling thealignment film to the product being bumped, and aligning the productbeing bumped; and shuttling the alignment film to the transfersubstrate, and aligning the transfer substrate.
 2. The method of claim1, further comprising the steps of: removing the alignment film; andshuttling the transfer substrate to the product being bumped.
 3. Themethod of claim 2, further comprising the steps of: urging the transfersubstrate against the product being bumped.
 4. The method of claim 3,further comprising the steps of: supplying heat to reflow solder ballsfrom the transfer substrate to the product being bumped.
 5. The methodof claim 4, further comprising the steps of: releasing the transfersubstrate.